Effect of particle size and pressure on the conversion of fuel N to NO in the boundary layer during devolatilization stage of combustion

The fate of fuel nitrogen evolved during pyrolysis in the boundary layer of a single particle is studied by modeling. The conservation equations for energy, species, and momentum are solved numerically in the boundary layer. The particles are treated as nonisothermal, because there can be great temperature gradients inside the particle, which affect the rates of devolatilization and nitrogen release. Global reaction mechanisms reported in literature are used for volatiles and nitrogen species in the gas phase. The fuel type (reactivity), particle size, gas temperature, oxygen concentration, and pressure affect the stoichiometric conditions in the boundary layer of a combusting particle and, consequently, the NO formation. The effect of particle size on the conversion of HCN and NH₃ to NO in the boundary layer is studied. As examples, particles burning in air at two gas temperatures 1350 and 1900 K are considered. For small particles, the pyrolysis and char combustion stages become overlapping, and therefore, the conversion of nitrogen in char and volatiles takes place simultaneously. The NO formation was found to decrease with increasing particle size, when the size exceeded a critical size. The experimentally reported trend for the decrease of NO formation with increasing pressure is shown by model calculations for particle (diameter 80 µm) burning in oxidizing gas containing 10.0 wt % O₂ at 1350 K.